Devices such as snubbers have been commonly applied in order to control dynamic excitation of piping systems from loads such as those caused by earthquakes. The intended function of these devices is to allow a pipe to move in order to accommodate normal operating conditions such as thermal movements, but to lock to prevent response to dynamic loading. Typically snubbers are either hydraulic or mechanical devices. Both types have experienced difficulties in service.
Mechanical snubbers often suffer damage to internal components which causes the snubbers to lock up and fail to accommodate free movement of the piping due to normal operational thermal cycling. This introduces the possibility of overloading of the piping and support system and the potential failure of the piping or the support system.
Hydraulic snubbers have the potential of leakage which could prevent them from adequately controlling the dynamic excitation of the piping. There is an additional record keeping complication with hydraulic snubbers because of the non-metallic components therein.
In nuclear power plants the history of failure of mechanical and hydraulic snubbers has led to a requirement for regularly scheduled inspection, testing and maintenance of all snubbers in a plant. These programs represent a significant financial burden to the owners of these power plants.
There have been efforts to provide simpler, more reliable substitute devices for snubbers in order to eliminate the costs of inspection, testing and maintenance programs. Among these approaches is the limit stop or gapped restraint. This type of device provides controlled limits within which the pipe can move freely to provide for normal operational movements but beyond which the pipe comes hard against a stop. It is generally true that when these devices bottom out due to dynamic loading, the impact loads on the device may be unacceptably high for the existing building structure.
Devices such as limit stops have been commonly applied in order to control static movement of piping systems from loading conditions such as those caused by temperature of the piping. The intended function of these devices is to allow a pipe to move to a predetermined point in order to accommodate normal operating conditions such as thermal movement, but to prevent movement beyond that point, in order to control or limit pipe stresses and loads on the supporting structure, such as turbine, boiler, and pump nozzles, to desired levels. Limit stops are commonly used in fossil power plants on critical piping where the pipe temperatures are very high (e.g., 10000.degree. F. or more) and the thermal pipe movement is large (e.g., 10 inches or more).
Limit stops used to date typically are made of structural steel members, and have the disadvantages of requiring excessive space in the plant, and of introducing friction loads into the piping and supporting structures. Particularly problematic are friction forces that result from axial pipe movement after the pipe has engaged the limit stop. Currently, designers must develop limit stops on a case-by-case basis. Frequently, the resulting limit stop designs introduce excessive, unpredictable loads on the equipment nozzles, since it is difficult to predict what pipe load is needed to overcome the frictional resistance of the steel frame.